Testing Optical Feeds in Wireless Systems

Many wireless equipment vendors are transitioning from coax cables to lower-loss optical feeds in Remote Radio Units (RRU) and Distributed Antenna Systems (DAS). The switchover is due to the fact that fiber optic cabling has less signal loss and supports higher bandwidth over greater distances than coax, providing a way to extend the distance from the amplifier or between components.

Working with Fiber – What You Need to Know If you are a technician who has worked mostly with copper, be aware that optical fiber requires very different tools and procedures. This is particularly true in regard to testing fiber optic cables and cleaning the connectors on optical feeds. This article will familiarize you with the general tools and procedures for working with fiber, as well as provide you with the specific steps for testing optical feeds. The examples below are based on the Ericsson RBS 3418/3518 radio base station with RRU 22 xx40 remote radio units. The process should be similar for other models and vendors as well as distributed antenna systems.

Supplies:

Step 1: Power Meter and Light Source

These devices connect to both ends of the fiber link and provide the most accurate measurement of optical power loss within the link or cable. They are typically the least expensive, however testing requires access to both ends of the fiber which may not be possible due to the mounting location of the RRU. They also cannot identify where the problem is located along the fiber link. For that, you’ll need the following device.

Step 2: OTDR

This device is the workhorse of optical testing. An OTDR uses signal reflectance, similar to radar technology, to graph fiber characteristics. An OTDR provides details about the characteristics of connectors, bending stress, etc., as well as the fiber itself. An OTDR is also a sophisticated fault locator that can identify the location and type of optical faults, even over great distances. An OTDR does not require access to both ends of a fiber for testing. OTDRs are not created equal! It is important to select an OTDR that features very high resolution, short dead zones and is easy to use. The Anritsu Network Master MT9090A shown in The Tool Box on page 22 in this magazine is one such tool.

Step 3: USB-Based Video Inspection Microscopes

These scopes provide a large image of the fiber endface within the connector ferrule, making this an effective tool for ensuring that the endface is clean and free of defects. The USB computer interface enables the image to be saved for review by managers or for inclusion in reports. Video scopes are also safe to use. Because the connector endface is viewed indirectly on a video screen, there is no danger of accidentally exposing the eye to harmful ultra violet (UV) light . Never view a fiber endface directly. UV light is invisible, so you won’t know if you are exposing yourself to harmful light.

Step 4: Miscellaneous Items

Working with fiber also requires various connector adaptors, cleaning supplies and fiber optic patch cords (jumpers) for connecting test equipment to the fiber.

Step 5: Locating Optical Feeds

With most radio base stations (RBS), the optical feed cable is a duplex cable that plugs directly into one of the sector ports of the transmission card. NOTE: there may be multiple RRU cables plugged into each RBS so care should be taken not to disconnect an RRU already in service. Proper cable labeling is recommended.

Step 6: Identifying Optical Connector Types

Several connector types are available for optical feeder cables. Each connector type is available in either SIMPLEX (one fiber) or DUPLEX (two fibers) configurations.

It is estimated that up to 70% of all errors in fiber optic systems are the result of dirty or damaged connectors. When dealing with optical fibers, connector condition and cleanliness are two of the most important factors. Dirty or damaged connectors will reduce the quality of transmissions and cause large amounts of light to be reflected back that may potentially damage transmitters. By using proper cleaning and inspection techniques, system turn up errors can be virtually eliminated and network reliability greatly increased.